DriveRack PA settings for T39s and OT212's

[Matrix]

Hello All,

My turn to tap into the collective.....

I'm running my first gig with BFM cabs [4 x T39 (20' with 3012's) and 4 OT212's]. It's a high school prom with about 500 kids in the high school gymnasium.

I will be running the subs on one amp out the DRPA low side and the tops on another out of the DRPA high side.

What are the suggested crossover settings for this cab combination on the DRPA? (frequency as well as crossover emulation...)

I'm hoping to wall load the subs but will not be able to corner load since I'm in the middle of one wall under one of the basketball goals....

Any other thoughts or suggestions are much appreciated!!

Thanks in advance!!

Doug

[DJPhatman]

You have to experiment with the crossover frequency, but between 100Hz and 125Hz is a good start. Start high, then lower it until you get the balance you like.

I kind of think you are going to over-power the subs, with 4 OT212. That's 8 mid drivers against 4 sub drivers. My guess is that you may only need 2 OT212 to cover the gym, or you will have to hold them back to keep a good balance.

Do V-plate! the extra dbs added will help.

Get set up as early as possible, and run the system to war volume. Check the gym, and make adjustments as needed, before the kids get there. Most of all, relax and have fun with it. How you handle the stress will show what a pro you are.

[Tim A]

I think 4 20" T-39's loaded with the 3012's will keep up just dandy if you have amps to take full advantage.

High pass the subs @ 45Hz if you want to run full tilt. High pass @ 40Hz if you want more lows, but limit them to half power. Use the steepest slope you have on the Driverack.

4 of those will make 131.6dB @ half power EQ'd flat to 40Hz, not V plated. 2 of the Otops per side will be right around 129dB EQ'd flat to 100Hz, again at half power. Sounds like a match made in heaven and WAY TOO DAMNED LOUD FOR A GYM!!!!!
(info courtesy of bgavin)

The OTop has a decided drop at 100Hz, so go no lower than that. There's not much volume to be gained by crossing higher, maybe a dB. Personaly, I like to start around 120Hz with my DR200's to help keep the lower frequencies out of the tops. Like DJPHat says, mess around with it and see where it sounds best.

[bgavin]

OP, your rig is the same as my outdoor rig: four OT212, four subs.
This is massive overkill for a gym, so you will have to turn down. A lot.

Locating along the wall at mid point is going to suck for acoustics.
Stacking the OT212 will give you some vertical pattern control. Very useful in a gym.
If you can do mono, a secure stack of four OT212 is a lot of pattern control.
Your optimum T39 is a 2x2 stack, V-Plated.
Splitting subs in a gym is a cancellation nightmare.

Set your DRPA to high pass at 46 Hz, 24dB/octave or better.
This will give you the requisite -3dB at 42 Hz, which is the half-power (225w) point for 3012LF in T39.
This setting will let you run the full 450w rating to each 3012LF, if needed.
Again, this is serious (and pointless) over driving of your gear.

The -3dB difference between running half power vs. full power isn't worth doing.
Your system will sound a LOT cleaner at half power, and run no risk of damage.
Doubling from four subs + four tops to eight of each, gives another (clean) 3dB. Very diminishing returns.

[Matrix]

Thanks Guys,

I will be running two Crown XLS802D amps (one on the subs and one on the tops). Per Crown they run 800W/channel at 4 Ohm with both channels driven.

If you think two OT212's will keep up with four T39's that would fix a problem I have......the OT's are 4 Ohm each and the 802 isn't rated for 2 Ohm......if I need to run 4 OT's then I'll need to drag out my old backup amp and then the setup get's more complicated (although I guess I could specify mono in the DRPA setup and then run one high output to each amp for the OT's).

Do you have a preference between the Butterworth or the Linkwitz-Riley filters?

Doug

[Matrix]

bgavin,

What would your placement of choice be in a gym? I may be able to request a move if I ask soon.....

Doug

[bgavin]

L-R is preferred, because it is more flat through the crossover region.
Most active 24dB/octave are L-R types.

I haven't played a gym since I was a kid. A long, long time ago.
The acoustics have not improved over the years. At all. Sucked then, suck now.

Link to Wiki and Starting Point

Another idea is reverse engineering the room, as you can do for a home theater setup.
Please your subs where the majority of the audience will be.
Generate pink noise in the sub passband.
Walk the perimeter with a measurement mic or device, and find the hot spot.
Locate your subs at the hot spot.

This will no doubt change somewhat when the place is full of people, but the drift is the same.
This may result in your subs located away from the tops, if the crowd remains in front of your console.

[Matrix]

You are so right about the acoustics of gyms!!!

One redeeming fact in this case is a false "ceiling" of draperies and crepe paper the decorating committee is going to hang about 10-15 feet above the floor. It's a start at least.....

Do you think two OT212's can hang with the four T39's??

Doug

[bgavin]

Absolutely.

[Dave Non-Zero]

The -3dB difference between running half power vs. full power isn't worth doing.
Your system will sound a LOT cleaner at half power, and run no risk of damage.
Doubling from four subs + four tops to eight of each, gives another (clean) 3dB. Very diminishing returns.

Max SPL may only go up by 3db but the nearfield advantage becomes significant. Thats the main reason i want to go to 4dr280 per side. Especially in a horrible acoustic environment like a gym.

[bgavin]

If I read this chart correctly, does a 2.0m stack work as nearfield at 5 meters distance?

[Tim A]

Down to about 850Hz @ 5 meters, farther in higher freqs. Nothing below 850.

[Tim A]

Down to about 850Hz @ 5 meters, farther in higher freqs. Nothing below 850.

It sounds like you are saying that the higher frequencies above 850hz keep their near-field drop off rate further than lower? Is that correct?

I thought outside, especially, the higher ones start falling off faster, and you sometime have to address that when tuning a system outdoors for longer "throws"?

Thanks!
Phil

As far as I know, nearfield = -3dB per doubling of distance, farfield = -6dB. Look at the chart, on a 2 meter array such as Bruce suggests, 850Hz has nearield advantage to about 5 meters. But higher frequencies retain the nearfield longer. For example, at about 1700Hz nearfield happens out to 10 meters. (hmmm...double the distance, one octave higher...I may be on to something!)

Directly porportional to the length of the array, and another reason those stupid piezo arrays work so well in these boxes.

I think what your referring to is how distance (air friction) affects higher frequencies more than lows. So the longer the array, the farther the nearfiled, the less loss.

Of course I may be all wrong, it's been known to happen.

[bgavin]

The higher frequencies attenuate at a much faster rate.
That is why infrasonic whale song travels hundreds of miles, where HF does not.

When I step back 300 feet or so from the stage, I can hear the T39s doing their thing quite clearly.
The highs are non-existent that far back, but quite apparent in close.

My concern about tall arrays is the beaming they do.
A straight array is well known for its "laser beam" directivity in the high frequencies.
I understand the beam width is about the same as the array height.

I can see where the J-array has use, but then you have to feather the output... yikes so much to do.

[Dave Non-Zero]

yes the HF is affected by air friction, which is why it is so important outdoors and for long throw applications to use a line array, because the nearfield effect at the higher frequencies counteracts that roll off to some extent. That graph is a theoretical description of the relationship between array height and nearfield effect, not measured.

here is an extract from a really interesting page about from Meyer about practical line arrays. It has absorbtion tables and stuff.

http://www.meyersound.com/support/paper ... theory.htm

'Nor does the purely theoretical computation reflect the reality of air absorption and its effects at high frequencies. The table below shows the attenuation at various distances from an array of 100 one-inch pistons spaced one inch apart, as modeled using a Bessel function. At 500 Hz and above, it also shows the total attenuation when air absorption is included using the calculation given in ANSI Standard S1.26-1995 (the conditions for this table are 20° C ambient temperature and 11% relative humidity). Note that, while at 16 kHz the array as modeled by the Bessel function is approaching 3 dB attenuation per doubling of distance, air absorption makes its actual behavior closer to 6 dB per distance doubling.'

As for aiming the array, Im sure Bill said that the array has the same vertical dispersion as one box. which for my DRs at least seems to be true. Ive not found it a problem to aim, and in difficult aoustics its a godsend! Dont be scared of the tight coverage, rejoice!